1. Field of the Invention
The present invention relates to a semiconductor wafer test equipment, and more particularly to an apparatus and method for setting zero point of Z-axis in a semiconductor wafer probe station.
2. Description of Related Art
At the end of a semiconductor fabrication process, a wafer test process is normally performed in order to sort good and bad or failed chips of the wafer by testing the electrical characteristics of each chip or die. Such a wafer test process is also called "Electrical Die Sorting (EDS)" and is carried out by test equipment including a tester and a probe station. The tester electrically determines the good and bad chips, and the probe station allows the wafer to move to the preset position for the test of each chip of the wafer.
Conventional wafer test equipment is illustrated in FIG. 1. The test equipment 10 includes a tester head 12, a performance board 16, and a probe card 18. The probe card 18 has probe needles or tips 19 to be in contact with the pad of chips of a wafer 20. The wafer 20 is loaded in top portion of a wafer chuck 22 of a probe station. The probe station enables the wafer chuck 22 to move in X/Y-axis direction at intervals of chip size and subsequently to move in Z-axis direction to contact the probe card tips 19 of the tester. Further, the wafer chuck 22 has vacuum pins 24, shown in FIG. 2, which hold the wafer 20 during the test process. The tester head 12 receives electrical signals from a tester system and supplies it to the probe card 18 via the performance board 16. The test signals are applied to a semiconductor device of each chip through the probe tips 19, thereby finding out if the device is operational. The above test operation is carried out by a test program.
Referring to FIG. 2, there is shown a top surface of the chuck 22 in which three vacuum pins 24 are provided through holes of the chuck top. The wafer is placed on a top portion ofthe vacuum pins 24. The vacuum pins 24 are arranged to move up and down along with Z-axis direction movement of the wafer chuck 22. Also, the vacuum pins 24 are connected with a vacuum line (not shown) to hold the wafer by a vacuum force during the wafer test process.
In the probe station, it is essential that the wafer chuck 22 should move precisely in the X/Y/Z-axis directions to perform the test operation. In particular, the Z-axis movement of the probe station is most important since it determines the contact face between the pad of the chip and the probe tips 19 of the tester. To initialize the probe station, the reset points (zero points) for the X/Y-axis direction movement have been set, while the zero point for the Z-axis is variable.
In order to set the zero point for the Z-axis movement, the top surface of the wafer chuck 22 moving upwardly along the Z-axis is arranged to be level with the top of the vacuum pins 24, and then the vacuum pins 24 are leveled up at a predetermined height above the chuck top. The height depends on the specification of a probe system and is set to, for example, 180 mils. Those two step adjustment operations are illustrated in FIG. 3A and 3B.
Therefore, adjustment of the zero point for the z-axis totally depends on leveling the chuck top with the vacuum pins. The leveling has been performed manually by the naked eye or the finger touch, with the result that an error is occurring at every adjustment. It has been found that the error covers minimum 3 mils and maximum 5 mils. A dial gauge or block gauge may be used to level the chuck top with the vacuum pins. However, it is difficult to use such a gauge in the wafer chuck top area. Furthermore, the measuring process is tedious and time-consuming.